1. Field of the Invention
The present invention relates to the field of safety warning devices and systems for construction, commercial and residential sites. More particularly, the present invention relates to warning devices and systems for warning construction workers, commercial building owners and home improvers of the dangers inherent in certain types of concrete slabs.
2. Description of the Related Art
Concrete, as used in the construction industry, is a mixture of one or more inert aggregate materials, a binding material such as portland cement, and water. The mixture is cast into a formwork, generally a wooden mold which holds the concrete during the pouring and curing stages. The slurry mixture then cures, hardening by a chemical reaction. In this manner, a cast concrete member of uniform thickness is produced. This cast member is generally referred to as a concrete slab.
Concrete alone is inherently incapable of resisting tension. For that reason, steel reinforcing is used to strengthen concrete. Reinforcing steel bars are known in the industry as rebars. These rebars reinforce the concrete by imparting the tensile characteristics of the reinforcing steel to the concrete. Thus, the bond between the concrete and the rebars must be sufficient to keep them from being pushed or pulled out of the slabs.
In unstable areas, and in situations where high loading tensions are expected, a stiffer beam is desired. For that reason, design and safety considerations often dictate the use of prestressed or post-tensioned concrete slabs. In such slabs, the presence of pre-applied compressive stresses in the tension region of a beam prior to normal loading will cause the beam to undergo less deflection once the intended load is applied. In other words, prestressing or post-tensioning concrete slabs produces stiffer beams. These stiffer beams greatly increase stability and safety of the structures erected therewith. Alternatively, smaller beams can be employed in place of larger and weaker ones, thus reducing both the cost and weight of the structures constructed.
In both prestressed and post-tensioned concrete slabs, steel tendons are laid down within the beam prior to pouring and curing of the concrete slab. In prestressed concrete slabs, tension is applied to the steel tendons prior to pouring and curing the slab. This is done by attaching the steel tendons to anchors and applying tension thereto. The concrete mixture is then poured within the formwork, and the concrete slab allowed to cure. The steel tendons are then severed, thereby transferring the tension to the concrete. In post-tensioned concrete slabs, the formwork is built with tubes running from end to end. The steel tendons are then inserted within the tubes, and the concrete poured. After the concrete cures, a desired tension is applied to the steel bars within their respective tubes. The tubes are then filled with, for example grout, or the terminal portions of the tendons are anchored to the slab in some manner. Alternatively, greased tendons may be used, the grease coating preventing the steel tendons from bonding to the concrete as it cures.
The tensions applied to the steel tendons, in either the prestressed or post-tensioned situations are great, on the order of tens of thousands of pounds of force. Tensioning forces on the order of 20,000 lbs. are typical. While this tensioning produces a stiff slab of concrete well suited to high stress building situations, use of pre or post-tensioned concrete slabs is not without dangers.
In particular, if the bond between the concrete and the steel tendons is reduced somehow, there exists the real danger that the forces applied to them will catastrophically release in an unintended manner. Alternatively, if the structural integrity of the steel tendons is compromised in any manner, the forces applied to the steel tendons can cause them to shoot out of the concrete slabs with great force. This can happen, for example, if the prestressed or post-tensioned concrete slab is drilled, or in any way penetrated, and the drill bit or penetrating element encounters one of the highly tensioned steel tendons. This can also happen if nails or bolts are driven into a pre or post-tensioned slab of concrete. The steel tendons can then whip unexpectedly out of the concrete slab with a force nearly equal to that applied to them when the slab was formed. The violent release of these tensioned steel tendons has been known to injure and kill people and damage property.
Thus, construction workers, homeowners, or commercial building owners who are unaware that tensioned concrete slabs have been used in their site, building or home are in grave danger of a potentially fatal injury should they, for any reason, drill, nail, drive a bolt into or through, or in any way penetrate the prestressed or post-tensioned concrete slabs. Even penetrating one inch in such slabs is highly dangerous, as the tensioning steel tendons often come to within an inch of the surface of the slab.
What is desired is a means for warning construction workers, commercial building owners, homeowners and all others of the presence of prestressed or post-tensioned concrete slabs within their job site, building or homes.
Conventional devices and methods are known which indicate the location of various electrical, communications and structural members embedded within cement or concrete slabs. One such device is disclosed in U.S. Pat. No. 2,854,840 to Anderson.
Anderson discloses a wire comprising a number of magnetic markers attached thereon. The wire and magnetic markers are embedded within cement floors and indicate the presence of power or communications lines therein. However, a compass is needed to detect these magnetic markers, rendering them invisible to the unaided eye. They are, as such, poorly suited as indicating or warning devices to warn, for example, construction workers at a job site of the presence of potentially dangerous prestressed or post-tensioned concrete slabs.
Another approach was taken by Bates, as disclosed in U.S. Pat. No. 5,003,735. In Bates, resilient fibers project from the surface of a poured concrete surface to indicate the presence of, for example, power lines, to help in locating potential electrical outlets. The resilient nature of the projecting fibers allows them to resiliently yield upon engagement by means employed in finishing the outer surface of the concrete or cement slab. It is apparent that these fibers, although resilient, will interfere with the various polishing processes used in finishing concrete or cement slabs. Indeed, these resilient fibers could get caught in the polishing machine, thus causing delays and further costs.
Palmer, in U.S. Pat. No. 1,531,754, discloses a device wherein an inscription plate is fastened to a concrete slab by inserting a bolt through a plate-and-anchor combination previously imbedded in the cement.
In the approach taken by Haggenjos in U.S. Pat. No. 1,624,048, an anchor is cast into the concrete and an inscription plate is applied afterward.
The devices set forth by Palmer and Haggenjos are not flush with the concrete surface and would not be suitable for prestressed or post-tensioned concrete slabs, as the outward projection of a bolt, plate, or other parts could interfere with the polishing of the concrete. Additionally, inserting a fastener into an anchor of unspecified length, such as described by Palmer, could cause the anchor to press outward and into the slab, creating just the kind of hazard which it is desired to avoid.
Kramer et al., in U.S. Pat. No. 4,979,462, disclose a device and a method for indicating the location and direction of various structural members in concrete members. In said patent of Kramer et al., an indicator device is disclosed which spans the width of the concrete slab, the indicator device having both top and bottom indicators. The disclosed indicator assembly consists of a post having a length substantially equal to the width of the concrete slab, the post supporting, at its extremities, the top and bottom indicators. Each of the top and bottom indicators are secured onto the posts by a retaining collar. Furthermore, the indicator assembly relies on a clamping assembly which is connected, via a sleeve arrangement, to the post such that the clamping assembly slides along the post. The clamping assembly, in turn, comprises coupling means having a slot in which the structural member, or strengthening cable, is to be inserted. The Kramer et al. device has many parts and is complicated, and the device of Kramer et al. requires different lengths of posts, one length for each different thickness of concrete slab utilized at the job site. Moreover, a great many number of tensioned steel tendons are used within many prestressed or post-tensioned concrete slabs. Often these steel tendons are laid crisscrossing-crossing each other, forming a checkerboard of steel tendons within the concrete slab. Therefore, to effectively warn people of the presence of all such steel tendons, a great many such devices are required, rendering the practical use of the Kramer et al. device unrealistic and impractical. In addition, each of the Kramer et al. devices must be installed and adjusted manually before the concrete is cast. By virtue of this, the device disclosed in the Kramer et al. patent is inherently expensive, and necessitates a large amount of time to set up properly. In addition, the Kramer et al. device is inherently non-removable, as it cannot be removed from the concrete slab once the slab has been cast. Thus the costs associated with utilizing such devices greatly increase, and the number of such devices necessary to indicate each individual reinforcing member unreasonably increases the time required to complete the casting of the concrete slabs. The extra time and cost necessary for use of the Kramer et al. device constitutes a disincentive to its use. Contractors bidding on projects will be unlikely to specify its use and insurance companies will also be unlikely to consider these indicator devices a statistically cost effective means for preventing injury and damage to property.
What is desired, therefore, is a warning device or system that is simple, has few or no moving parts, is easy to install, has the potential to be installed by machine rather than manually, is cost-effective for the purchaser, and is highly visible. What is also desired is a warning device which does not compromise the integrity of the prestressed or post-tensioned concrete slab in which it is installed. What is further desired is a removable warning device or system that can be installed during construction and removed after construction if the device is placed in a location where the prestressed or post-tensioned slabs are unlikely to be disturbed. What is additionally desired is a warning device or system which will not interfere with the polishing or other further processing and use of the concrete slab in which the warning device is installed. What is desired, furthermore, is a warning device or system that is inexpensive to manufacture, is easily installed, is removable, and which will effectively communicate the presence of prestressed or post-tensioned concrete slabs.
It is an object of the present invention to provide a simple warning device and system for warning construction workers, commercial building owners and homeowners of the presence of prestressed or post-tensioned concrete slabs within their job site, building, home or any other structure.
It is another object of the present invention to provide a warning device and system that is simple, has few parts, is inexpensive to manufacture and is quick to install.
It is a further object of the present invention to provide a warning device and system that is securely attachable, durable, highly visible and removable.
In accordance with the above objects and those that will be mentioned and will become apparent below, a stress-tension hazard indicating device for indicating the presence of prestressed and post-tensioned concrete members comprises:
a plate member with a front surface and a back surface, and which is designed to be installed in a partially cured concrete slab such that the front surface of the plate member will be flush with the concrete surface when the concrete surface is fully cured;
a visible indication on the front surface of the plate member, the visible indication signifying the presence of a hazard;
at least one through hole in the plate member and/or at least one projection in the rear surface of the plate member, which is used to removably attach the plate member to at least one concrete penetrating projection;
at least one device for removably connecting the plate member to at least one concrete penetrating projection, either via a through hole in the plate member or via a projection in the rear surface of the plate member, such that the connecting device is flush with or under the plate member after the assembly is installed in concrete;
at least one concrete penetrating projection having a proximal and a distal end, a portion at the distal end which is flared or otherwise configured such that the concrete penetrating projection will be securely anchored in a cured concrete slab after prior installation in the partially cured slab, and the proximal end being attached to the back surface of the plate member with at least one connecting device which is flush with the surface of the plate member or below the plate member;
whereby, a concrete member is provided with a flush tendon hazard warning indicator from a time no later than the time at which the concrete member is fully cured.
In another embodiment of the present invention, the stress-tension hazard indicating system, comprises a plate member having a front surface, the plate member having at least one through hole, a visible indication on the front surface, the visible indication communicating a presence of a prestressed or post-tensioned slab of concrete, at least one anchor member; and at least one fastener, the fastener being removably insertable through respective ones of the at least one through hole into respective ones of the at least one concrete anchor, whereby, when the at least one concrete anchor is embedded within a partially cured prestressed or post-tensioned concrete slab and remains therein as the slab cures, the plate member is removably attachable to the prestressed or post-tensioned concrete slab by inserting the at least one removable fastener through the at least one through hole into the at least one concrete anchor.